Restarting oil and gas wells closed by the extreme cold sweeping much of the U.S. isn’t going to be quick or easy, even once the ice thaws and power is restored. Oil production nationwide has been cut by at least a third and in the Permian Basin of Texas, the heart of America’s shale industry, output has plummeted by as much as 65%. But bringing flows back is likely to take much longer than it took for them to slump.

Water can still cause problems even after it’s been separated from the valuable hydrocarbons. If the pipes carrying waste water away freeze, you can’t get rid of the water from the well and you have to shut the whole operation.

Warmer weather will help. With temperatures forecast to rise this weekend, the wells and pipelines should begin to thaw naturally. That will allow oil and gas to start flowing again, but that’s only the start of the process.

Some well sites could have experienced mechanical damage, with freezing water expanding to crack valves, compressors, and pipes.

“There’s a good bit of water produced with oil and gas, so in areas with higher water, you’re going to have burst pipes,” according to Jim Newman, Executive VP of Operations at Basic Energy Services. “Safety will be paramount because of the hydraulics being damaged. So there’s going to be a very meticulous reactivation.”